tests: blob_ui 39.8s -> 4.1s, and a fatal it was hiding

Three separate wastes, all of the same family: waiting on a guess instead of a
signal, and paying for work that never had to be repeated.

1. The 64KB blob was built with os.urandom into a fresh TemporaryDirectory on
   every run. New bytes at a new path means the pristine-database cache can
   never apply, so full auto-analysis of 64KB of AArch64-decoded noise was paid
   every single run. It is now built from a seeded PRNG at a stable path
   (tests/.synthetic/, gitignored) and staged through the existing cache.

   Determinism is also a correctness fix: whether 64KB of chance bytes contains
   something IDA reads as a function is luck, and this suite asserts "and really
   has no functions".

2. `wait(lambda: lst.model is not old, ..., 30)` after commenting. The perf work
   made an item edit KEEP the listing's walk and re-render in place, so the
   model object is never replaced and this waited out its full 30s timeout on
   every run -- and then "commenting leaves the view where it was" passed
   vacuously, because nothing had happened at all. A test that burns 30s to
   check nothing is worse than no test.

3. Two `pause(2.0)`/`pause(2.5)` after a carve, replaced with settle() on a real
   condition. The second one deliberately has NO predicate: that spot is random
   data, so the carve may legitimately produce nothing, and "the row became
   code" would never hold -- gating on it cost another 30s timeout. What that
   check is about is the VIEW not moving, so the gate is "the app finished
   reacting".

Fixing (1) exposed a real bug in the client, fixed here too: reopening a
database that already exists while passing loader switches is FATAL in IDA --

    FATAL ERROR: Switch '-b400' can be used only when loading a new file

which kills the worker before it can report anything. Loader switches describe
an IMPORT and are recorded in the database they produce, so they are now sent
only when there is an import to describe. This was never reachable from the old
suite (a fresh random blob never had a database to reopen), but it is reachable
by any user who opens a raw blob with --ida-args twice.

30 passed, 0 failed.

0 files changed, 0 insertions, 0 deletions